The present invention relates generally to paraboloidal antenna systems, and more specifically the invention pertains to a means for maintaining a paraboloidal surface contour which is required for optimum performance of large aperture-to-wavelength ratio antennas.
Communication antennas often make use of an antenna dish, which provides a wide surface for capturing radio frequency signals. Such antennas usually have a paraboloidal shape. The advantage of the paraboloidal contour is that signals arriving parallel with its axis of symmetry are reflected to the focal point of the paraboloid, where a primary feed or pickup probe is located.
The maximum diameter-to-wavelength ratio, at which large microwave and millimeter wave reflectors produce acceptable gain and radiation patterns, depends on the precision of the paraboloidal contour as manufactured and retained under environmental stress. For terrestrial applications, massive mechanical backup structures are generally required to support paraboloidal surfaces of 1000 wavelengths or more in diameter against deformation due to gravity, wind loading, and other forces. For spaceborne applications, thermal stresses and lack of rigidity of unfurlable designs set a lower limit to the wavelength at which they can be used.
High gain, space-borne satellite communication antennas must accurately maintain their gains to provide the required link margins. Lightweight spacebased antennas are particularly vulnerable to erection induced deformations. The impact of these errors becomes increasingly more severe as the operating frequency increases. Thus, correction techniques such as this invention will become more and more important in future systems. Large groundbased antennas operating at frequencies up to and exceeding 1OO GHz will also need such alignment systems to maintain performance.
Certain aspects of the task of reducing the need for structural rigidity of large paraboloidal antenna reflector systems by providing a means for dynamically correcting errors in the paraboloidal contour are included, to some extent, by the systems disclosed in the following U.S. Patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 4,825,223 issued to Moore; PA1 U.S. Pat. No. 4,710,777 issued to Halverson; PA1 U.S. Pat. No. 4,482,897 issued to Dragone et al; PA1 U.S. Pat. No. 4,458,251 issued to Bandon; PA1 U.S. Pat. No. 4,811,033 issued to Ahl et al; PA1 U.S. Pat. No. 4,660,941 issued to Hattori et al.
The patents identified above relate to reflectors and antennas. In particular, the Moore patent describes a reflective assembly of paraboloidal surfaces, each individually but rigidly aligned, so that microwave signals impinging on any of the surfaces are reflected onto one common focal point.
Halverson discloses a dish antenna structure which uses reinforced inner ribs to strengthen the dish shape and limit its flexibiltity.
The Dragone et al patent describes an antenna with a segmented reflecting surface. The segmentation of the reflecting surface provides for separate images of the far field area of the antenna on separate focal surfaces. This is the reverse of what is intended by the subject invention. It proves, though, that individual panels can be aligned such that they focus onto desired points.
The Bandon patent relates to a paraboloidal microwave reflector, which can be assembled from a plurality of identical and interchangeable rigid fiberglass panels. To assure thermal stability the panels are supported by ribs. The ribs form a mounting ring and incorporate self-indexing devices for automatic alignment of the panel front surfaces.
The Ahl et al patent discloses a system for controlling the surface contour of a deployable and restorable antenna. The antenna, when deployed, forms a paraboloidal reflector surface. The Ahl et al disclosure attains its objective by spacially deforming the single continuous reflector surface through appropriately placed external forces, rather than by optimally aligning an otherwise ideal set of paraboloidal subsurfaces.
A method for angular alignment of such ideal paraboloidal subsurfaces is presented in the Hattori et al patent. Dual stacks of piezoelectric transducers provide orthogonal tilt to a flat optical mirror surface.
In a journal article by J. Nelson, entitled "The Keck Telescope," published in American Scientist, 1989, Vol. 77, pp. 170-176, an optical 10 m reflector is described, composed of 36 hexagonal precision segments. These segments are arranged in a mosaic and their positions actively controlled to create a single continuous optical surface. Active position control is accomplished by sets of two capacitive sensors on every intersegment edge. Readings of all intersegment relative positions are interpreted by computer and position adjustment commands are issued to three actuators attached to each segment. Three actuators suffice to adjust the position and inclination of each segment and hence achieve a continuous and optimally aligned optical surface.
While the above-cited references are instructive, the task remains to provide an antenna reflector system, which is composed of segments, which can be individually adjusted to conform to a true paraboloidal surface, by referring to an absolute system of reference. The present invention is intended to satisfy that need.